In the extrusion of peroxide cured thermoplastics and thermoplastic elastomer compositions an accumulation of material often appears at the face (“front”) of the die. This accumulated material is often referred to as die “drool” or die “mustache”. Die lip buildup (or “mustache”) poses a significant problem for extruder operators. An effort to describe the causes of die lip buildup in conventional thermoplastic extrusion is made by Jessie D. Gander and A. Jeffrey Giacomin in “Review of Die Lip Buildup in thermoplastics Extrusion” 37(7)POLYMER ENGINEERING AND SCIENCE (July, 1997). Though this reference refers to thermoplastics extrusion, it is believed to be an apt reference with respect to the cause of die mustache in thermoplastic vulcanizates (“TPV”) upon extrusion. The shape, and more specifically, the consistency of the shape, imparted to the molten TPV as it passes through the extruder die is dependant on the smoothness of the die at its mouth. The presence of material deposits or buildup at the mouth of the extruder die can affect the shape of the die mouth. Additionally, deposits may impart unwanted grooves on the extrusion as the deposit digs into the surface of the molten extrusion. Finally, deposits may, from time to time, slough off of the die lip and onto the TPV extrusion, creating an unwanted bead or glob on the extrusion.
Methods to reduce die mustache have generally produced unsatisfactory results, either in the total reduction of die lip buildup, or in the overall physical properties of the resultant TPV. Two approaches are described in EP 0 911 364 A1 and EP 1 550 693 A1. In the first reference, a process for making a TPV is described that involves adding a portion of the total polyolefin thermoplastic to the TPV composition after the elastomer curing agent has been added to the extrusion chamber and the elastomer has been substantially cured. This approach requires extra extrusion steps and is thus not desirable. In the latter reference (EP 1 550 693 A1), syndiotactic polypropylene is substituted for isotactic polypropylene as the preferred polyolefin thermoplastic. While reduced die buildup is claimed, the resultant TPV has poor compression data at the extrusion temperature.
Other references of interest include EP 0 436 724 A1 and U.S. Pat. No. 6,720,376.
It would be desirable to provide a method of forming thermoplastic vulcanizates that result in a reduction of die mustache as compared to conventionally formed TPVs while maintaining other physical properties.